PCI: endpoint: Fix configfs group removal on driver teardown

PCI: endpoint: Fix configfs group removal on driver teardown

An endpoint driver configfs attributes group is added to the
epf_group list of struct pci_epf_driver by pci_epf_add_cfs() but an
added group is not removed from this list when the attribute group is
unregistered with pci_ep_cfs_remove_epf_group().

Add the missing list_del() call in pci_ep_cfs_remove_epf_group()
to correctly remove the attribute group from the driver list.

With this change, once the loop over all attribute groups in
pci_epf_remove_cfs() completes, the driver epf_group list should be
empty. Add a WARN_ON() to make sure of that.

Fixes: ef1433f717a2 ("PCI: endpoint: Create configfs entry for each pci_epf_device_id table entry")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20250624114544.342159-3-dlemoal@kernel.org